Wireless communication systems are assuming ever-increasing importance for the transmission of data, which is to be understood in its largest sense as covering speech or other sounds and images, for example, as well as abstract digital signals.
Currently proposed standards for wireless communication systems between a stationary base station and a number of remote (mobile or immobile) stations include the 3GPP (3rd generation Partnership Project) and 3GPP2 standards, which use Frequency Division Duplex (‘FDD’) or Time Division Duplex (‘TDD’) and Code Division Multiple Access (‘CDMA’). The HIPERLAN and HIPERLAN2 local area network standards of the European Telecommunications Standards Institute (‘ETSI’), use Time Division Duplex (‘TDD’) and Orthogonal Frequency Division Multiplex (‘OFDM’). The International Telecommunications Union (‘ITU’) IMT-2000 standards also use various multiplex techniques of these kinds. The present invention is applicable to systems of these kinds and other wireless communication systems.
In order to improve the communication capacity of the systems while reducing the sensitivity of the systems to noise and interference and limiting the power of the transmissions, various techniques are used separately or in combination, including space diversity, where the same data is transmitted over different physical paths interleaved in time, in particular over different transmit and/or receive antenna elements, and frequency spreading where the same data is spread over different channels distinguished by their sub-carrier frequency.
At the receiver, the detection of the symbols is performed utilising knowledge of the complex channel attenuation and phase shifts: the Channel State Information (‘CSI’). The Channel State Information is obtained at the receiver by measuring the value of pilot signals transmitted together with the data from the transmitter. The knowledge of the channel enables the received signals to be processed jointly according to the Maximum Ratio Combining technique, in which the received signal is multiplied by the Hermitian transpose of the estimated channel transfer matrix.
Two broad ways of managing the transmit diversity have been categorised as ‘closed loop’ and ‘open loop’.
Two closed loop methods are described in the paper entitled “Transmit adaptive array without user-specific pilot for 3G CDMA” by B. Raghothaman et al., that appeared in the IEEE Transactions 2000. In the systems described in this paper, the signals transmitted over the different transmit antenna elements of the base station are weighted according to relative weights calculated at the receiver from Channel State Information and retransmitted to the transmitter. In one system referred to, pilots specific to each user are transmitted in addition to the pilots for each transmit antenna element that are common to all users, which penalises the communication capacity of the system. In another system disclosed in the paper, user-specific pilots are avoided by re-modulating the detected signals using the measured Channel State Information and the calculated weights and using the re-modulated signals to correct errors in feedback; this imposes a heavy computational load on the receiver and the result is only reliable if the channel state estimation is sufficiently correlated with the actual channel state to avoid a high detection error rate.
In pure ‘open loop’ methods, no Channel State Information is fed back to the transmitter. In such systems, the transmitter comprises a plurality of transmit antenna elements; the data is encoded in symbol blocks, the symbols of a block being permuted between the transmit antenna elements over time with respective replications and complex conjugations and/or negations. The complexity of the receiver depends on the properties of the matrix that defines this space-time block code; in particular detection is performed with a low cost in terms of simplicity of the receiver computations if this matrix is an orthogonal one. Orthogonal matrices are well known: definitions are given in textbooks such as ‘Matrix Computations’ by Gene H. Golub and Charles F. Van Loan, 3rd Edition, published by Johns Hopkins. See page 69 (for a set of vectors) or page 208 (for a matrix).
An open loop system using an orthogonal detection matrix is described in International Patent Application Publication No WO 99/14871 Alamouti. In this system, the symbols of a block transmitted are permuted between the transmit antenna elements over time with respective replications and complex conjugations and/or negations according to a scheme, known as the ‘Alamouti code’, such that the received signal is detectable at the receiver using an orthogonal detection matrix scheme.
The performance of the code is mainly based on the diversity order of the code. This diversity order characterizes the number of transmit and receive antennas which is actually seen by the code. For a given number of receive antenna elements, the more transmit antenna elements are used the more improvement is obtained in terms of fading and interference is obtained. However, the paper entitled “Space-Time Block Codes from Orthogonal Designs” by V. Tarokh et al. that appeared in IEEE Transactions on IT, vol. 45, Jul. 1999, states that an orthogonal detection code matrix can not be used if the transmitter comprises more than two transmit antenna elements with full diversity without sacrificing the coding rate, that is to say the useful data rate for the user. They propose coding rates of ½ for three to eight transmit antenna elements or ¾ for three or four transmit antenna elements.
Patent specification WO 00/51265, Whinnett et al., assigned to Motorola, describes another transmit diversity system, in which code rate is maintained for arrays of more than two transmit antenna elements but at the expense of sub-optimal transmit diversity.
Another transmit diversity scheme (ABBA code) is described for more than two transmit antenna elements in the paper entitled “Minimal Non-Orthogonality Rate 1 Space-time Block Code for 3+Tx Antennas” by O. Tirkkonen et al. IEEE 6th Int. Symp. On Spread-Spectrum Tech. & Appli., NJIT, pp. 429-432, September 2000. This coding rate 1 scheme is derived from the permutation of two Alamouti codes as described by the code matrix
         [                            A                          B                                      B                          A                      ]  It is stated that the ABEA code provides full spatial diversity to the detriment of the orthogonality of the detection matrix, which implies that the computational cost of the detection step is increased compared to an orthogonal scheme. In addition the performance of the ideal code is not fully achieved by the ABBA code due to the interference terms of the detection matrix.
Other compromises are proposed in a paper presented by H. Jafarkhani to the IEEE Wireless Communications and Networking Conference in September 2000 with non-orthogonal detection matrices that are stated not to achieve simultaneously the optimum diversity and transmission rate, two encoding schemes proposed being of the kind described by the code matrices
      [                            A                          B                                                  B            *                                                -                          A              *                                            ]    ⁢          ⁢            and      ⁢                          [                                    A                                B                                                              -                              B                *                                                                        A              *                                          ]        .  
Yet another compromise is described in the paper “A randomization technique for non-orthogonal space-time code blocks” by A Hottinen et al. appearing in IEEE VTC 2001. However, this system still does not employ an orthogonal detection matrix with full diversity for more than two transmit antenna elements.
Still another compromise is described in the paper “A space-time coding approach for systems employing four transmit antennas” by C. B. Papadias et al. presented at an IEEE conference in 2001 and that proposes an encoding scheme of the kind
      [                                        b            1                                                b            2            *                                                b            3                                                b            4            *                                                            b            2                                                -                          b              1              *                                                            -                          b              4                                                            b            3            *                                                            b            3                                                b            4            *                                                -                          b              1                                                            -                          b              2              *                                                                        b            4                                                -                          b              3              *                                                            b            2                                                -                          b              1              *                                            ]    .This scheme also uses a non-orthogonal detection matrix that does not achieve simultaneously the optimum diversity and transmission rate.